1. Field of the Invention
The present invention relates to a so-called active matrix-type display device represented by a liquid crystal display device provided with a plurality of pixels which respectively have switching elements, an electro-luminescence-type display device or a display device provided with a plurality of pixels which respectively have light emitting elements such as light emitting diodes, and more particularly to a blanking process of a display image in a hold-type display device.
2. Description of the Related Art
A liquid crystal display device has been popularly used as a display device in which images based on image data (video signals in case of television broadcasting) inputted from outside in accordance with every frame period are displayed by holding respective brightness of a plurality of pixels which are arranged two-dimensionally at a desired value within a given period (1 frame period, for example).
In the liquid crystal display device adopting an active matrix scheme, as shown in FIG. 9, pixel electrodes PX and switching elements (thin film transistors) SW which supply video signals to these pixel electrodes PX are formed on a plurality of respective pixels PIX which are arranged two dimensionally or in a matrix array. A die on which a plurality of pixels PIX are arranged is also referred to as a pixels array 101, wherein the pixels array in the liquid crystal display device is also referred to as a liquid crystal display panel. In this pixels array, a plurality of pixels PIX constitute a so-called screen which displays images.
In the pixels array 101 shown in FIG. 9, a plurality of gate lines 10 (also referred to as scanning signal lines) which extend in the lateral direction and a plurality of data lines 12 (also referred to as video signal lines) which extend in the longitudinal direction (the direction which crosses the gate lines 10) are juxtaposed respectively. As shown in FIG. 9, along the respective gate lines 10 which are respectively discriminated by addresses consisting of G1, G2, . . . Gj, Gj+1 . . . Gn, so-called pixel rows in which a plurality of pixels PIX are arranged in the lateral direction are formed. On the other hand, along the respective data lines 12 which are discriminated by addresses consisting of D1R, D1G, D1B . . . DmB, so-called pixel columns in which a plurality of pixels PIX are arranged in the longitudinal direction are formed. The gate lines 10 apply voltage signals which are supplied from a scanning driver 103 (also referred to as a scanning driving circuit) to the switching elements SW provided to the respective pixels PIX which constitute the pixel rows (arranged below the respective gate lines in the case shown in FIG. 9) which respectively correspond to the gate lines 10, and open or close the electrical connection between the pixel electrode PX mounted on each pixel PIX and one of data lines 12. An operation to control a group of switching elements SW provided to the specific pixel row by applying voltage signals to the group from the gate lines 10 which correspond to the group is also referred to as “selecting line(s)” or “scanning”. The above-mentioned voltage signals applied to the gate lines 10 from the scanning driver 103 are also referred to as scanning signals and, for example, control the conductive state of the switching elements SW in response to pulses generated in signal waveforms thereof. Further, in response to kinds of switching elements SW, the scanning signals are supplied to the scanning signal lines (corresponding to the gate lines 10) as current signals.
On the other hand, to the respective data lines 12, display signals (voltage signals in case of the liquid crystal display device) which are referred to as gray scale voltages (or tone voltages) are applied from a data driver (also referred to as video signal driving circuit) 102. The above-mentioned gray scale voltages are applied to the respective pixel electrodes PX selected by the above-mentioned scanning signals on the pixels PIX which constitute the pixel columns (right side of respective data lines 12 in case of FIG. 9) which correspond to respective display signals.
When such a liquid crystal display device is incorporated into a television set, with respect to 1 field period of video data (for video signals) received by an interlace mode or with respect to 1 frame period of video data received in a progressive mode, the above-mentioned scanning signals are sequentially applied to the gate lines 10 in the order from the addresses G1 to Gn, and the gray scale voltages generated based on video data received during 1 field period or 1 frame period are sequentially applied to a group of pixels which constitute the respective pixel rows. In each pixel, a so-called capacitive element is formed by sandwiching a liquid crystal layer LC between the above-mentioned pixel electrode PX and a counter electrode CT to which a reference voltage or a common voltage is applied through a signal line 11 and the light transmissivity of the liquid crystal layer LC is controlled based on an electric field generated between the pixel electrode PX and the counter electrode CT. As mentioned above, when the operation to sequentially select the gate line G1 to the gate line Gn is performed one time in accordance with every field period or every frame period of the video data, for example, the gray scale voltage applied to the pixel electrode PX of a certain pixel during a certain field period is theoretically held in the pixel electrode PX until the pixel electrode PX receives another gray scale voltage in the next field period which follows the certain field period. Accordingly, the light transmissivity of the liquid crystal layer LC sandwiched between the pixel electrode PX and the above-mentioned counter electrode CT (in other words, brightness of the pixel which includes this pixel electrode PX) is held in a given state in accordance with 1 field period. The liquid crystal display device which displays images by holding the brightness of the pixel in accordance with every field period or in accordance with every frame period is also referred to as a hold-type display device. This hold-type display device is classified from a so-called impulse-type display device such as a cathode-ray tube which illuminates phosphors provided to respective pixels by the irradiation of electron beams at the moment that the device receives video signals.
Video data which is transmitted from a television receiver set or, a computer or the like has a format corresponding to the impulse-type display device. To compare a driving method of the above-mentioned liquid crystal display device with television broadcasting, the scanning signals are applied for every gate line 10 within a time which corresponds to the inverse number of horizontal scanning frequency of the television broadcasting and the applying of scanning signals to the whole gate lines G1 to Gn is completed within time corresponding to the inverse number of the vertical frequency. Although the impulse-type display device sequentially illuminates the pixels which are arranged in the lateral direction on the screen like impulses in accordance with every horizontal scanning period in response to horizontal synchronizing pulses, the hold-type display device, as mentioned above, selects the pixel rows in accordance with horizontal scanning period and supplies the voltage signals simultaneously to a plurality of pixels included in the pixel row and holds the voltage signals to these pixels after the completion of the horizontal scanning period.
Although the manner of operation of the hold-type display device has been explained by taking the liquid crystal display device as an example in conjunction with FIG. 9, an electro-luminescence type (EL type) display element which replaces the liquid crystal layer LC with an electro-luminescence material or a light-emitting diode array type display device which replaces the capacitive element sandwiching the liquid crystal layer LC between the pixel electrode PX and the counter electrode CT with a light emitting diode is also operated as the hold-type display device although they differ in an operation principle (images being displayed by controlling a carrier injection amount into a light emitting material). In the display device which generates images by injecting the carrier into the light emitting material (light emitting regions), the above-mentioned display signals are supplied to respective pixels within the pixels array as current signals.
Here, the hold-type display device displays images by holding the respective brightness of the pixels in accordance with the above-mentioned every frame period, for example. Accordingly, when the display image is replaced with another display image between a pair of continuing frame periods, there arises a case that the brightness of the pixel does not sufficiently respond. This phenomenon is explained such that the pixel which is set to a given brightness during a certain frame period (first frame period, for example) holds the brightness corresponding to the first frame period until the pixel is scanned in the next frame period (second frame period, for example). Further, this phenomenon can be also explained in view of a so-called hysteresis of video signals of respective pixels. That is, a portion of the voltage signal (or carrier injected to the voltage signals) which is transmitted to the pixel within the first frame period interferes with the voltage signal (or carrier to be injected to the voltage signal) to be transmitted to the pixel during the second frame period. Techniques which solves such problems related with responsiveness of the image display in the display device using the hold-type light emitting are disclosed in Japanese Accepted Patent Publication 016223/1994, Japanese Accepted Patent Publication 044670/1995, Japanese Laid-open Patent Publication 073005/1993, Japanese Laid-open Patent Publication 109921/1999 and Japanese Laid-open Patent Publication 166280/2001 respectively.
Among these publications, Japanese Laid-open Patent Publication 109921/1999 discusses a so-called blurring phenomenon in which when moving images are reproduced using a liquid crystal display device (an example of a display device using hold-type light emitting), a profile of an object becomes indefinite compared to a cathode ray tube which illuminates pixels as impulses. To solve such a blurring phenomenon, Japanese Laid-open Patent Publication 109921/1999 discloses a liquid crystal display device in which a pixels array (a group of a plurality of pixels which are arranged two dimensionally) of one liquid crystal display panel is divided into upper and lower portions of a screen (image display area) and data line driving circuits are respectively provided to the divided upper and lower pixels arrays. The liquid crystal display device performs a so-called dual scanning operation in which one gate line is selected from each one of upper and lower pixels arrays, that is, two gate lines in total with respect to the upper and lower pixels arrays are selected and video signals are supplied from data line driving circuits formed on the respective pixels arrays. While performing this dual scanning operation within 1 frame period, a vertical phase between upper and lower pixels arrays is shifted such that signals corresponding to display images (so-called video signals) are inputted to one pixels array from one data line driving circuit and signals of blanking images (black images, for example) are inputted to the other pixels array from the other data line driving circuit. Accordingly, a period for performing the video display and a period for performing the blanking display are applied to both of the upper and lower pixels arrays during 1 frame period so that a period in which the videos are held over the whole screen can be shortened. Accordingly, the liquid crystal display device can also obtain the moving image display function comparable to that of the cathode ray tube.
That is, as the conventional technique, Japanese Laid-open Patent Publication 109921/1999 discloses a technique in which one liquid crystal display panel is divided into two upper and lower pixels arrays, the data line driving circuits are respectively formed on the divided pixels arrays, two gate lines in total consisting of one gate line for the upper pixels array and one gate line for the lower pixels array are selected so as to perform the dual scanning of the upper and lower display regions divided in halves using respective driving circuits, and the blanking images (black images) is interpolated by shifting the vertical phase within 1 frame period. That is, 1 frame period can take the state of the video display period and the state of blanking period so that the video holding period can be shortened. Accordingly, with the use of the liquid crystal display, it is possible to obtain the moving image display function of the impulse-type light emitting as in the case of the cathode ray tube.
On the other hand, another technique which suppresses a blurring phenomenon of moving images displayed by a liquid crystal display device is disclosed in Japanese Laid-open Patent Publication 166280/2001. This publication discloses a driving method of a liquid crystal display device. In this driving method, a period for selecting gate lines for supplying video signals to a group of pixels corresponding to respective gate lines is divided, wherein to a group of pixels corresponding to the gate lines selected in the former half of the selection period, video signals are supplied, while to another group of pixels which correspond to another gate lines selected in the latter half, voltage signals which perform a black display of such another group of pixels are supplied. The summary of this driving method of the liquid crystal display device is explained in conjunction with an example in which the pixels array shown in FIG. 9 is driven in accordance with a timing chart shown in FIG. 10. In accordance with every frame period, the gate lines G1, G2, . . . Gj, . . . Gj+1 in the pixels array 101 are respectively selected in response to gate pulses (also referred to as gate selection pulses) generated by scanning signals transmitted from the scanning driver 103. In other words, the switching element SW which is provided to each pixel PIX corresponding to the gate line which receives the gate pulse assumes a state in which a display signal O-DDR transmitted from the data line 12 in response to the gate pulse is received by the pixel PIX. For example, in response to outputting from the data driver 102 of the display signal L1 generated by one line of the video data to be supplied to a group of pixels (also referred to as pixel rows since pixels are arranged in a row direction) corresponding to the gate line G1, the gate line G1 is selected in response to the gate pulse. FIG. 10 indicates a gate pulse as a waveform which makes the scanning signal of Low state shifted to High state, wherein over a period in which the scanning signal assumes the High state, the gate line which receives this scanning signal is selected.
In a driving method of a liquid crystal display device disclosed in Japanese Laid-open Patent Publication 166280/2001, to supply a display signal (any one of L1, L2, Lj, Lj+1, . . . in FIG. 10) corresponding to one line of video data to respective pixel rows, out of time tg which selects the gate line (G1, G2, Gj, Gj+1 in FIG. 10) corresponding to the display signal, a time tb which constitutes the latter half is allocated to the selection of a separate gate line (gate line Gj with respect to the gate line G1) and a display signal (B in FIG. 10) which displays a pixel row corresponding to the separate gate line in black is supplied to the pixel row. The gate line which is selected within a time (tg–tb) and in which the video data for one line is written and the gate line which is selected within time tb and in which black data (corresponding to display signals which displays pixels in black) is written are selected such that they are spaced apart from each other in the pixels array. Due to such a constitution, by completing the formation of images due to writing of video data into the pixels array and the cancellation of images in accordance with every frame period, the images are formed on a screen in the same manner as an impulse-type display device and the blurring of moving images can be reduced.
To compare the liquid crystal display device described in the above-mentioned Japanese Laid-open Patent Publication 109921/1999 and the liquid crystal display device described in the above-mentioned Japanese Laid-open Patent Publication 166280/2001, the former can selects two gate lines simultaneously and supplies the display signal corresponding to the video data for one line to the pixel row corresponding to one gate line and the display signal which displays the pixel row corresponding to the other gate line in black to the other pixel row. Due to such a constitution, it is possible to ensure time for supplying the display signal to respective pixels which constitute respective pixel rows. However, in one frame period, a period in which the pixel row holds the display signal corresponding to the video data is limited to one half of the frame period. Accordingly, in particular, when the delay time is necessary for the brightness of the pixel to reach a value corresponding to the display signal from the supply of the display signal, there arises a problem that the display device receives a next display signal which displays the pixel in black before the pixel acquires the sufficient brightness. To solve this problem, it is necessary to increase the intensity of the display signal and this inevitably necessitates the increase of the output of the data driver 102. Further, as mentioned above, in the liquid crystal display device described in Japanese Laid-open Patent Publication 109921/1999, the pixels array is divided into two regions and hence, it is inevitably necessary to provide data line driving circuits respectively. Accordingly, the liquid crystal display panel and circuits in the periphery of the liquid crystal display panel naturally become complicated and large-sized.
On the other hand, the liquid crystal display device described in Japanese Laid-open Patent Publication 166280/2001 is more practical than the liquid crystal display device described in Japanese Laid-open Patent Publication 109921/1999 in view of the structure and size of the liquid crystal display panel and peripheral circuits thereof. However, as can be clearly understood from a timing chart in FIG. 10, a portion of the gate line selection period for writing the video data for one line to the pixel row is allocated to the selection of the separate gate line for writing black data to the separate pixel row and hence, the presence of the problem that a time for supplying the display signals to respective pixel rows becomes short cannot be denied. In SID 01 Digest (The 2001 International Symposium of the Society for Information Display), pages 994 to 997, a technique which can solve the above-mentioned problem of the liquid crystal display device in Japanese Laid-open Patent Publication 166280/2001 is described. To explain this technique in conjunction with FIG. 10, the ratio of the time tb with respect to the time tg is suppressed to tg/2 so as to ensure the video data writing time to the pixel rows. On the other hand, the black data writing to the pixel rows is repeated in response to the video data writing to the pixel rows in plural times so as to replenish the shortage of the writing time tb for one time. Accordingly, the writing of black data to the gate lines Gj, Gj+2, Gj+4, . . . (latter two not shown in FIG. 10) with respect to the writing of the video data to the gate line G1 and the writing of black data to the gate lines Gj+1, Gj+3, Gj+5, . . . (latter two gate lines not shown in FIG. 10) with respect to the writing of the video data to the gate line G2 are respectively performed.
In this manner, although the black data writing time to the gate lines can be ensured as a sum of writing times, the shortage of time for every black data writing is insufficient to compensate for the delay of brightness response of the pixel. That is, compared to the pixel which receives the sufficient display signal in the black data writing to the gate line at a time, the pixel which receives the display signal divided in a plural times exhibits the slow brightness response. Accordingly, the display signal of video data to be erased remains in the pixel even after the writing of black data is started and hence, the possibility that the erasure of image based on the video data from the screen which is to be completed in one frame period becomes unfinished to the contrary cannot be denied.
It is an object of the present invention to provide a display device and a method for driving the display device which can suppress a blurring phenomenon of moving images displayed on the display device and can sufficiently hold the display brightness while minimizing the structural change of a periphery of pixels array of a hold-type display device represented by a liquid crystal display device.